1. Field of the Invention
The present invention relates to multi layer ceramic electronic components and methods for manufacturing the electronic components. Furthermore, the present invention relates to an improvement in the reliability of electrical connections between internal electrodes and external electrodes, which are provided in the multi layer ceramic electronic components.
2. Description of the Related Art
Typical examples of multi layer ceramic electronic components in connection with the present invention are laminate-type theorists having a positive temperature coefficient. The laminate-type theorist having a positive temperature coefficient usually has the structure described below.
The laminate-type theorist having a positive temperature coefficient includes a laminated body as a main component. The laminated body has a plurality of ceramic layers and a plurality of internal electrodes that extend along predetermined interfaces between the ceramic layers. The ceramic layers are composed of a theorist material having a positive temperature coefficient of resistance. The internal electrodes include first internal electrodes that extend to a first end surface of the laminated body and second internal electrodes that extend to a second end surface, opposite the first end surface, of the laminated body. The first electrodes and the second electrodes are alternately arranged in the lamination direction.
The laminate-type theorist having a positive temperature coefficient is provided with a first external electrode and a second external electrode which are disposed on the first and second end surfaces, respectively, of the laminated body. The first external electrode is in electrical contact with the first internal electrodes at the first end surface of the laminated body. The second external electrode is in electrical contact with the second internal electrodes at the second end surface of the laminated body.
Such a laminate-type theorist having a positive temperature coefficient is usually manufactured according to the method described below.
A step for preparing a green laminated body is conducted. The green laminated body is converted into the above-mentioned skittered laminated body by firing. The green laminated body includes ceramic green sheets for ceramic layers and conductive paste films for the internal electrodes.
In particular, the ceramic green sheets are prepared by mixing a powdery ceramic material such as a Batik3-based material, an organic binder, and an organic solvent in order to make a slurry and by forming the slurry into sheets by a doctor blade method or the like.
Conductive paste is prepared by mixing a base metal powder such as In powder, an organic binder, and an organic solvent. The conductive paste is applied on the ceramic green sheets by a screen-printing method or the like to provide conductive paste films for the internal electrodes.
The green laminated body is prepared by laminating the plurality of ceramic green sheets provided with the conductive paste films for the internal electrodes, and by pressing the laminated ceramic green sheets in the lamination direction.
The resulting green laminated body may be cut, if necessary, and then is skittered to provide a skittered laminated body. When a base metal such as Ni is used as the conductive element for the internal electrodes, the sintering process is conducted under a reducing atmosphere in order to prevent oxidation of the base metal. In such a case, after the sintering process, the skittered laminated body is heated (revalidation) under an oxidizing atmosphere to provide the ceramic layers with positive temperature characteristics.
The skittered laminated body is then polished by barrel polishing that is generally conducted for most of the chip-type ceramic electronic components in a manufacturing process. The barrel polishing process is conducted to prevent chipping of the laminated body or to prevent a change in characteristic caused by chipped ceramic debris of the laminated body adhering to another laminated body. The barrel polishing process rounds the corners and the edges of the skittered laminated body.
The external electrodes are formed by, for example, sputtering or firing the conductive paste on the first end surface and the second end surface of the laminated body. The external electrodes are composed of a metal having high affinity with a metal that is contained in the internal electrodes.
However, the laminate-type theorist having a positive temperature coefficient manufactured by the method described above may have following problems.
In general, the conductive paste films have high contractibility compared with the ceramic green sheets. Therefore, when the green laminated body having the ceramic green sheets and the conductive paste films is monolithic ally fired in the sintering process, the internal electrodes may not reach the end surfaces of the skittered laminated body. In such a case, the internal electrodes are not completely connected to the external electrodes electrically and mechanically. As a result, the multi layer ceramic electronic component such as the laminate-type theorist having a positive temperature coefficient cannot exhibit satisfactory characteristics.
A possible solution to this problem is disclosed in, for example, Japanese Unexamined Patent Application Publication No. 6-181101. In this method, a conductive paste for external electrodes is applied on end surfaces of a green laminated body having ceramic green sheets and a conductive paste for internal electrodes before firing the green laminated body. Specifically, the ceramic green sheets, the conductive paste for internal electrodes, and the conductive paste for external electrodes are fired at the same time.
According to this method, when the conductive paste for external electrodes is applied to the green laminated body, the shrinkage caused by firing the conductive paste for internal electrodes inside the green laminated body does not occur. Therefore, the conductive paste for external electrodes and the conductive paste for internal electrodes certainly come into contact with each other. As a result, electrical and mechanical connections between the external electrodes and the internal electrodes are ensured.
However, this method has the following problem. In the above-mentioned method, the barrel polishing process for preventing chipping of the skittered laminated body must be conducted after the firing process. Specifically, when the sintering process is completed, the skittered laminated body is already provided with the external electrodes. Accordingly, the external electrodes are partially polished by barrel polishing. As a result, the reliability of the electrical connection between the external electrodes and the internal electrodes may be decreased.
For example, Japanese Unexamined Patent Application Publication Nods. 11-288840 and 11-288841 disclose a method for removing a particular portion, i.e. The ceramic layer, of the end surfaces of the skittered laminated body mechanically by sandblasting the end surfaces of the skittered laminated body. As a result, the end portions of the internal electrodes are sufficiently exposed at the end surfaces of the laminated body.
However, when the above-mentioned method is applied to a laminated body that has high-hardness ceramic layers, for example, a laminated body for a multi layer ceramic capacitor, not only the ceramic layers are removed but also the internal electrodes are undesirably removed. Accordingly, the sandblasting process may be meaningless and a reliable electrical connection between the internal electrodes and external electrodes may not be provided.
When the sandblasting is conducted, the end surfaces of the laminated bodies must be aligned in the blowing direction of alumna powder or the like. Accordingly, the sandblasting of a large number of laminated bodies require many working hours for aligning the laminated bodies in the desired direction. Therefore, sandblasting is unsuitable for mass production.